    [Patent document 1] JP 2005-289175 A
There is a vehicle outputting a small traveling sound such as a vehicle driven on electric motor. Such a vehicle is not easily noticed by a pedestrian, posing a problem. To that end, there has been proposed a technology to cause a vehicular dynamic speaker mounted in a vehicle to output a notice sound such as a false engine sound towards an outside of the vehicle for announcing the presence of the vehicle in a vicinity of the vehicle (refer to Patent document 1). The above dynamic speaker is defined as directly emitting an audible sound.
The frequency range peculiar to a real engine sound is from about 4 Hz to about 250 kHz, for instance. If modeling after the real engine sound, a false engine sound is needed to have frequency levels comparable to those of the real engine sound in the frequency range from about 4 Hz to about 250 kHz.
Then, it has been proposed that the false engine sound having frequency levels approximately comparable to those of the real engine sound be outputted using a dynamic speaker. In such a case, in order to reproduce a low-pitched sound range of 800 Hz or less of the false engine sound, it is further proposed to mount a dynamic speaker adopting a large-sized cone type vibrating plate such as a woofer in a vehicle. In this regard, however, such a proposal poses a problem A1, such as increasing a difficulty in attachment in the vehicle, increasing a weight of the vehicle, or increasing costs.
In contrast, instead of the technology using a dynamic speaker, it may be proposed a technology that generates a notice sound such as a false engine sound outside of a vehicle using a parametric speaker device containing an ultrasonic wave speaker so as to announce the presence of the vehicle (it is not a well-known technology). The technology of generating a notice sound such as a false engine sound with a parametric speaker device is achieved by applying an ultrasonic modulation to a waveform signal of a notice sound such as a false engine sound to radiate via an ultrasonic wave speaker. The ultrasonic wave (sonic wave not heard by an ear of a human being) radiated from the ultrasonic wave speaker contains amplitude components, which will undergo a self-demodulation in the air in a course of the transmission or propagation. The notice sound is thereby generated (i.e., become audible) at a position distant from the vehicle.
Thus, the use of the parametric speaker device enables the radiation of the ultrasonic waves of which amplitude components generate the low-pitched sound. Therefore, the above problem (A1) can be avoided by using the parametric speaker device.
It is noted that a low-pitched sound reproduced by the parametric speaker device has a longer wavelength; thus, a delay is caused in the self-demodulation in the air while even though it belongs to a low frequency wave, a strong directionality (i.e., having a strong tendency in advancing straight) is exhibited unlike a low-pitched sound reproduced by a dynamic speaker that directly emits or outputs an audible sound. This poses another problem A2. That is, as the frequency decreases, a sound pressure of a frequency, which a person perceives, decreases depending on the direction of an external ear (earhole) of the person in a region near the ultrasonic wave speaker of the parametric speaker device, as illustrated as a solid line A in FIG. 12.
The solid line A in FIG. 12 indicates a frequency characteristic under the condition that a sound detection section of a microphone is arranged in an inner portion of a dummy external ear in a dummy head (modeling after a head portion of a human being) and detects a reproduced sound by the parametric speaker device. In addition, the broken line B in FIG. 12 is a comparative line that indicates a frequency characteristic under the condition that a microphone directly detects a reproduced sound by the parametric speaker device without using a dummy head. The comparison between the solid line A and the broken line B shows a clear tendency of the decrease in the sound pressure perceived by a person in the low-pitched sound range due to the influence of the external ear with the decrease of the frequency.
Thus, in the case where the parametric speaker device is used, the following takes place. That is, due to the above problem A2, a shortage of the low-pitched sound arises in a false engine sound reproduced by the parametric speaker device in a vicinity of the vehicle. This provides a hindrance to the purpose of making the false engine sound using the parametric speaker device have frequency levels identical to those of the real engine sound.
Furthermore, when a notice sound having a larger sound pressure is emitted from the dynamic speaker mounted in the vehicle, a vehicle occupant who is in the vehicle continuously has to hear the notice sound emitted from the dynamic speaker for a long time depending on the sound pressure. Therefore, it is necessary to lower the sound pressure level audible to the vehicle occupant below the limit or threshold value (for example, 65 dB) of the vehicle occupant's sensitivity.
To that end, if reducing an output level of the dynamic speaker in order to lower a sound pressure of the notice sound which the vehicle occupant hears down to the limit of the vehicle occupant's sensitivity, the propagation distance of the notice sound becomes shorter, making it impossible to send the notice sound to the target propagation distance.